[FEATURE][Haskell] Add Middleware support for the haskell servant generator

[Fjolnir-Dvorak]

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Description of the PR

I added middleware support to the haskell servant generator and extended the example to handle the additional middleware parameter.

[jonschoning]

reviewers/cc: @gibiansky @f-f @mandrean

[f-f]

I'd say instead of changing the signature of this function we could add a run{{title}}ServerWithMiddleware function, and provide the default implementation of the middleware here

In this way we wouldn't have to document how to get a default middleware, since we'd have that already implemented

[Fjolnir-Dvorak]

That would be a good Idea to keep backwards-compability, though I would keep the documentation since I think that the call with middleware should be the preferred one since an ID-function does not add any runtime overhead.

[Drezil]

In this way we wouldn't have to document how to get a default middleware, since we'd have that already implemented

The 'default'-Middleware is literally just id and the example-server would have that as default.
I fear that if one adds wrappers for this it could start to set a precedence for every case where this could be extended.

Another "clean" solution could be to add a datatype:

data ServerConfig = ServerConfig { serverMiddleware :: Middleware }
  deriving (....)

defaultServerConfig = ServerConfig id
-- or with data-default
instance Default ServerConfig where
  def = ServerConfig id

and people using defaultServerConfig would not be affected if we change things in any way in the future. Especially if the use the Default-Pattern: http://hackage.haskell.org/package/data-default

let myConfig = defaultServerConfig { serverMiddleware = myMiddleware }
-- or with data-default
let myConfig = def { serverMiddleware = myMiddleware }

This way we would never have to update/break the type-signature of the implementation for all future versions.